CONCENTRATION OF ACID WHEY USING FORWARD OSMOSIS

A Project Paper

Presented to the Faculty of the Graduate School

of Cornell University

In Partial Fulfillment of the Requirements for the Degree of

Master of Professional Studies in Agriculture and Life Sciences

Field of Food Science and Technology

by

Jer Lin Poh

August 2016

© 2016 Jer Lin Poh

ABSTRACT

The disposal of acid whey poses a significant problem for the dairy industry, due to its

high organic matter content, high acidity, and the large volumes produced. This dairy byproduct

contains lactose, protein, minerals, and lactic acid that can be utilized in the production of

various chemicals and food products. Acid whey can also be concentrated to reduce storage and

transportation costs, and to improve its shelf-life and stability. In this project, forward osmosis

(FO) was evaluated as a means of concentrating acid whey from the production of skim Greek

acid whey. Physicochemical analyses were performed to determine the pH, Brix, color, and

conductivity of the FO feed solutions and the concentrates obtained. A decrease in flux with time

was observed during all concentration runs, which was due to the increase in the total soluble

solids content in the feed solution. Raw acid whey was concentrated from 6.2 ± 0.1 ºBrix to 28.2

± 1.7 ºBrix using FO in 165 min. FO was also performed on pre-concentrated acid whey, which

was concentrated from 28.8 ± 0.7 ºBrix to 42.5 ± 3.3 ºBrix in 35 min. This study showed that FO

has real promise as a method of whey concentration that does not use excessive energy and

preserves well the quality of the concentrated product.

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BIOGRAPHICAL SKETCH

Jer Lin Poh was born and raised in Singapore. In 2012, she received a scholarship from

the Agri-Food and Veterinary Authority of Singapore to pursue her studies abroad. Before

attending graduate school at Cornell, she obtained a Bachelor of Science in Food Science at

McGill University in 2015. She conducted summer research at the National University of

Singapore in 2012, and has co-authored a paper in Food Chemistry.

She is interested in food product development and has led teams winning first place in the

2015 IFTSA & MARS Product Development Competition, as well as second place in the 2015

Chinese Institute of Food Science & Technology Global Product Development Competition and

the 2016 American Society of Baking Product Development Competition.

Jer Lin will be working in food regulation at the Agri-Food and Veterinary Authority of

Singapore after graduation. Eventually, she hopes to pursue a career in food product

development, or in research and development.

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I will like to dedicate this work to my family and friends

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ACKNOWLEDGMENTS

I would like to thank my advisor, Dr. Carmen Moraru, and her advice and support throughout the

course of my Master’s degree. I also wish to express my gratitude to Dr. Olga Padilla-Zakour,

Kyle Kriner, Malcolm Brooks, and John Churey for their assistance with my project.

Additionally, I would like to thank my labmates for their help and wonderful company: Yifan

Cheng, Emily Griep, Sheena Hilton, Jacqueline Morales, Pedro Menchik, Shaun Sim, Fan Wang,

and Jiai Zhang.

Finally, I would like to acknowledge my gratitude to the Agri-Food and Veterinary Authority of

Singapore for funding my undergraduate and graduate studies.

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TABLE OF CONTENTS

BIOGRAPHICAL SKETCH………………………………………………………….….….… iii

ACKNOWLEDGEMENTS ……………………………………………………...…..….......…. v

TABLE OF CONTENTS………………………………………………………...…………..… vi

CHAPTER 1: FORWARD OSMOSIS - FUNDAMENTALS AND APPLICATIONS...…. 1

1.1. INTRODUCTION……………………………………….……………….………… 1

1.2. PROCESS FUNDAMENTALS ………………………………………….….…..... 2

1.3. CONCENTRATION POLARIZATION…………………...……………….….….. 3

1.4. PROCESS PARAMETERS……………………………………………………..… 5

1.5. CHALLENGES……………………………….……………………………...…... 14

1.6. FOOD APPLICATINS OF FORWARD OSMOSIS………………………..….... 17

1.7. REFERENCES……………………………….………………………………..…. 24

CHAPTER 2: FORWARD OSMOSIS AS A NONTHERMAL METHOD

OF ACID WHEY CONCENTRATION ………………………………………………...…. 31

2.1. INTRODUCTION……………………………………….……………….…….… 31

2.2. MATERIALS AND METHODS ………………………………………….….….. 33

2.3. RESULTS AND DISCUSSION…………………………...……………….….…. 38

2.4. CONCLUSIONS…………………………………………………….….……….... 47

2.5. REFERENCES……………………………….………………………………..…...47

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CHAPTER 1

FORWARD OSMOSIS: FUNDAMENTALS AND APPLICATIONS

1.1. Introduction

Conventional osmosis involves the net transport of water from a solution of higher water

chemical potential to a solution of lower chemical potential, across a selectively permeable

membrane that allows the passage of water but is impermeable to practically all solutes and

ions. Osmotic pressure (π) is the amount of pressure that needs to be applied to the more

concentrated solution to prevent water flow across a semi-permeable membrane. Unlike

pressure-driven membrane processes like reverse osmosis (RO) which depend on the application

of hydraulic pressure, forward osmosis (FO) is exclusively driven by the osmotic pressure

gradient across the membrane. It requires a highly concentrated draw solution to produce a high

osmotic pressure that spontaneously drives water flow from the feed solution to the draw

solution. In the process, the feed solution is concentrated and the draw solution is diluted, while

maintaining a high rejection rate of contaminants.

Although the water flux achieved in FO is only approximately 20% that of RO, the process does

not require pre-filtration of the feed nor the application of hydraulic pressure. (Raghavarao,

Madhusudhan, Hrishikesh Tavanandi & Niranjan, 2014). The primary energy requirement in FO

is for the regeneration of the draw solution, and the overall energy cost of the process can be

significantly lower than that pressure-driven membrane processes if the draw solution can be

easily recovered or discarded. Furthermore, as lower pressures are involved, FO equipment tends

to be simpler in design, require less membrane support, and have a lower propensity for

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membrane fouling. The physical and organoleptic properties of the feed are also maintained as

FO does not require the use of heat or pressure, making the process suitable for food and

pharmaceutical applications. Appropriate membrane designs, draw solutions, operation

conditions should be selected to optimize the FO process, and will be discussed in the review.

1.2. Process fundamentals

In the absence of concentration polarization and reverse solute flux, water flux (Jw) in FO can be

expressed by Eq. 1.1:

, , (1.1)

where A is the membrane water permeability coefficient, πD,b is the bulk osmotic pressure of the

draw solution, and F,b is the bulk osmotic pressure of the feed solution.

It has been observed that experimental water flux are lower than predicted theoretical values.

This is because the osmotic pressure gradient across the active membrane layer is generally

lower than the bulk osmotic pressure differential, due to the occurrence of internal and external

concentration polarization (ICP and ECP). The extent of these phenomena depend of the physical

properties of the feed and draw solutes, fluid dynamics of the draw and feed solutions, as well as

the structure of the membrane.

McCutcheon & Elimenech (2006b) derived an equation above account for the effects of external

and internal concentration polarization, which will be discussed below:

, exp , exp (1.2)

where k is the mass transfer coefficient and K is the solute resistivity for diffusion within the

porous support layer.

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1.3. Concentration polarization

1.3.1 External concentration polarization

FO uses asymmetric membranes, which are composed of a porous support layer and a dense

membrane active layer. The draw solution may face either the support or active layer.

Concentrative ECP occurs when the feed solution flows across the active layer of the membrane,

causing a build-up of solute on the feed side of the membrane. On the other hand, dilutive ECP

occurs when the draw solution on the permeate side of the membrane is being diluted by the

permeating water. As a result, there is a reduction in the effective osmotic pressure gradient and

water flux across the membrane. ECP can be minimized by increasing flow velocity and

turbulence at the surface of the membrane, and by using a membrane layer in which the feed

solution faces the active membrane layer. It can also be limited by reducing the water flux, at the

expense of efficiency.

1.3.2 Internal concentration polarization

Lower-than-expected flux values in FO are mainly attributed to the occurrence of ICP in the

membrane support layer. In fact, the prevalence of ICP in the support layer is considered to be

one of the main drawbacks of FO, and can cause reductions in flux of up to 80%. In a study on

sucrose concentration using FO, it was determined that water flux does not increase

proportionally with the bulk osmotic pressure difference due to the presence of dilutive internal

concentration polarization. (Garcia-Castello, McCutcheon & Elimelech, 2009).

When the feed solution is aligned with the membrane support layer, as with the case of pressure-

retarded osmosis, concentrative ICP occurs as solutes in the feed solution build up within the

membrane support layer. On the other hand, when the draw solution is aligned with the

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membrane support layer, dilutive ICP occurs within the latter as water crosses the membrane into

the draw solution.

Unlike ECP, ICP cannot be mitigated by increasing the crossflow velocity or turbulence as it

occurs within the porous support layer. A possible solution to reducing ICP would be to develop

an FO membrane with a supportive mesh attached to the selective polymer layer, Instead of a

fabric support layer. (McCutcheon, McGinnis & Elimelech, 2005). Tang, She, Lay, Wang & Fe

(2010) found that at higher membrane fluxes and/or draw solution concentrations, ICP holds a

critical role in flux behavior because it depends exponentially on the flux level.

Figure 1.1. Illustrations of driving force profiles, expressed as water chemical potential, μw, for osmosis through

several membrane types and orientations. (a) A symmetric dense membrane. (b) An asymmetric membrane with the

porous support layer facing the feed solution; the profile illustrates concentrative internal CP. (c) An asymmetric

membrane with the dense active layer facing the feed solution; the profile illustrates dilutive internal CP. The actual

(effective) driving force is represented by μw. External CP effects on the driving force are assumed to be negligible

in this diagram. (McCutcheon, McGinnis & Elimelech, 2006b).

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1.4. Process parameters

1.4.1 Membrane orientation

The feed solution may face either the support or active layer. The former configuration is used

when the feed solution contains compounds of low molecular weight (e.g. water). On the other

hand, when the feed solution is composed of a complex mixture of substances (e.g. liquid food),

it is aligned with the active layer to prevent ECP. As such, a higher flux through the membrane

can be achieved with negligible damage to the membrane. (Sant'Anna, Marczak & Tessaro,

2012).

1.4.2. FO Membranes

Before 2000, most FO research used dense RO membranes. These have since been deemed to be

unsuitable for FO, as their thick support layers are more susceptible to ICP and fouling. FO

membranes can be classified by their method of fabrication, and include cellulose-based, thin

film composite (TFC), and chemically modified membranes. A comprehensive review of FO

membranes has been written by Zhao, Zou, Tang and Mulcahy (2012).

FO membranes are typically asymmetric with a low molecular weight cut-off of approximately

100 Da. Suitable materials for FO membranes include cellulose acetate, cellulose diacetate,

cellulose triacetate, polyamide, and polysulfone. (Raghavarao, Madhusudhan, Hrishikesh

Tavanandi & Niranjan, 2014). The active layer, which is responsible for rejecting solutes, should

have a hydrophilic and very thin active layer to permit high water flux. Flux increases when the

thickness of the membrane support layer is decreased, and has been ascribed to the decreased

resistance between the draw and feed solutions. (Petrotos, Quantick & Petropakis,1998; Dova,

Petrotos & Lazarides, 2007). The active layer should also have a high density to enable high

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solute rejection. The pore size of a conventional FO TFC membrane has been evaluated by Fang,

Bian, Bi & Wang (2014), and the pore radius distribution was found to be approximately skewed

to the right, with a mean pore radius was found to range between 0.25nm and 0.30nm.

The support layer, which supplies mechanical strength, should also be hydrophilic and have

minimum porosity to prevent ICP. (Xu, Peng, Tang, Fu & Nie, 2010; McCutcheon & Elimenech,

2007). The membrane should also be compatible with both the feed and draw solutions, have a

low susceptibility to fouling, and be capable to withstanding the mechanical stresses imposed by

the FO process.

In general, high water permeability in FO membranes comes at the expense of low salt rejection.

(Wei, Liu, Qiu, Wang & Tang, 2011). The specific reverse solute flux has been developed to

assess membrane selectivity, and is equal to the ratio of the solute flux in the reverse direction to

the forward water flux (Hancock & Cath, 2009). Two main types of FO membranes will be

described below: cellulosic and TFC membranes.

Cellulosic membranes are mainly formed using conventional phase inversion, often using

cellulose acetate as the dip coating, followed by hot water annealing. Aside from meeting the

aforementioned criteria, cellulose acetate is widely available and exhibits good resistance to to

chlorine degradation. However, it has poor resistance to hydrolysis and biological attachment. To

minimize hydrolysis, the pH of the feed and draw solutions must be maintained within 4 and 6,

and the temperature should be kept below 35◦C. Furthermore, cellulose acetate membranes

exhibit poor water permeability and selectivity. (McCutcheon & Elimelech, 2006).

To reduce ICP in the support layer, several cellulose ester-based FO membranes containing two

selective skin layers have been developed. (Wang, Ong & Chung, 2010). In these membranes,

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the dense layer facing the draw solution is used to reject solutes, while the layer facing the feed

solution reduces ICP by blocks fouling agents from entering the porous support layer between

the skins. However, a lower water flux was obtained. Double-skinned membranes have been

optimized in a model developed by Tang et al. (2011).

TFC membranes exhibit high salt rejection, high mechanical strength, and are chemically stable.

Unlike RO TFC membranes, which contain a thick support layer that can withstand the high

pressures involved in the process, TFC membranes designed for FO have a thinner support layer

which reduces ICP. Two types of TFC FO membranes are commercially available. The first type

of TFC membrane contains an embedded mesh for mechanical support, which is sandwiched

between a thin selective layer and a relatively loose support layer. This membrane has been

extensively used in many FO studies. The second type of TFC membrane contains a polymeric

skin layer and a hydrophilic support fabric, with a porous scaffold layer in the middle. While

both membranes are constructed of cellulose triacetate, the latter permits a higher salt rejection

rate, at the expense of a lower transmembrane flux. (Zhao, Zou, Tang & Mulcahy, 2012; Herron,

2008)

1.4.3 Membrane modules & devices

FO operations can be batch or continuous. In batch operations, the draw solution is diluted once

and is not reconcentrated for reuse, and the device is typically disposable. Examples of batch FO

applications include hydration bags for water purification, as well as osmosis pumps for drug

delivery. In continuous FO operations, the draw solution is continuously reconcentrated and

reused. The feed and draw solutions are recirculated on the feed side and the permeate side of the

membrane respectively.

8

Flat sheet membranes are the most widely available FO membrane modules, and can be used in a

plate-and-frame or spiral-wound configuration. Flat sheet membranes are thinner, which enables

a higher flux to be achieved. Tubular membrane modules (tubes and hollow fiber) have a higher

packing density, but are prone to excessive fouling and membrane integrity problems. However,

they are self-supported and do not need a support layer, which reduces concentration

polarization. Tubular membrane configurations are typically operated using turbulent flow

conditions, which also reduces concentration polarization.

Typical module configurations for forward osmosis include plate and frame, tubular, spiral

wound, and hollow fiber. Flat sheet or tubular membranes are typically used for laboratory-scale

purposes, while larger-scale applications usually employ flat sheet membranes in plate-and-

frame configurations.

In plate-and-frame module configurations, flat membrane sheets are sealed to frames, which

provide mechanical support. The system is typically immersed in a tank containing the feed

solution, and the draw solution is circulated between the membranes and plate support. Aside

from having insufficient membrane support and low packing density, other disadvantages of the

plate-and-frame configuration include unreliable sealing, as well as difficulty in ensuring the

integrity of the membrane.

Figure 1.2. Plate and frame module configuration.

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While commercial spiral-wound configurations are operated with only one stream, it can be

modified for FO usage, as shown in Figure 1.3. In this design by Mehta (1982), the feed solution

travels through the first half of the perforated tube, flows into the membrane envelope, and then

travels through the second half of the pipe. (Cath, Childress & Elimenech, 2006). It is only suited

for feed solutions with a low fouling propensity as backwashing of the channels cannot be

performed. Advantages of this system include its low capital cost and energy usage.

Figure 1.3. Spiral-wound module. (Cath, Childress & Elimenech, 2006).

Aside from the advantages mentioned above, tubular (tubes or hollow fiber) membranes are

more durable than flat sheet membranes. Hollow fibers have an internal diameter of 1 mm, while

tubular membranes have an internal diameter equal to or more than 2 mm. Only laminar flow can

be achieved in hollow fiber, while turbulence can be achieved in tubular membranes. As such,

less ECP, fouling, and scaling occurs in tubular membranes

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Figure 1.4. Tubular membrane configuration. (Nicoll, 2013).

1.4.4 Draw solutions: Composition, concentration

Selection of an appropriate draw solution is key is to the performance of FO. The draw solution

must have a higher osmotic pressure than the feed solution. A draw solute with a high osmotic

efficiency is needed to produce a high water flux, namely compounds with a high solubility in

water. As predicted by the van’t Hoff equation, multivalent ionic solutes with a high degree of

dissociation are also preferred as they yield solutions with a higher osmotic pressure. The draw

solution should also be non-toxic, chemically compatible with the membrane used, and be cost-

effective. Recovery of the draw solute from the permeate should be easy and inexpensive, as this

affects the energy requirements and operation costs of the process. If re-concentration is used to

regenerate the draw solution, it is critical that the draw solute is highly soluble in water to avoid

scaling during thermal evaporation. As draw solutes with low diffusion coefficients would

aggravate ICP, it may be preferable to use a draw solute with a high diffusion coefficient-

namely, one with a low molecular weight and viscosity in aqueous solution. However, such

solutes may aggravate reverse solute diffusion.

It is also critical to consider the feed solution and when selecting a draw solute. For instance,

when FO is used for the concentration of proteins or pharmaceuticals, it is critical to choose a

11

draw solute that will not degrade or denature the products as reverse solute diffusion may occur.

A comprehensive review of FO draw solutions was recently published by Ge, Ling & Chung

(2013). Sodium chloride is often used as a draw solute, and is easy to regenerate using RO or

distillation without risk of scaling. Other widely used draw solutes include CaCl2, Ca(NO3)2,

glucose, sucrose, as well as a thermolytic NH4HCO3 draw solution, which will be discussed later.

Most draw solutes can be classified into four categories: volatile compounds, nutrient

compounds, inorganic salts, and organic salts and polymers.

Volatile draw solutes can be separated from the diluted draw solution through heating or

distillation. The volatile gases are subsequently dissolved into solution to regenerate the draw

solution. The use of highly concentrated ammonium bicarbonate solution as a FO draw solution

has been found to yield extremely high osmotic driving forces. It can be decomposed into

ammonia and carbon dioxide gases by heating to 60C, and then recovered to regenerate the

draw solution.

Various inorganic salts have been used in FO. In general, they yield high flux rates and can be

easily recovered using RO. These include thermally recoverable salts, fertilizers, and others.

When fertilizer is used as the draw solute, the diluted draw solutions can be used for irrigation,

eliminating the draw solute recovery step and hence reducing energy costs. However, they may

not be compatible with some membranes as most chemical fertilizers form acidic solutions.

Furthermore, some fertilizers are only partly soluble in water or may not fully dissociate in

solution, and may not yield sufficient high osmotic pressures.

In a study by Achilli et al. (2010), 500 inorganic compounds were screened, from which 14

compounds were tested in an FO process. CaCl2, KHCO3, MgCl2, MgSO4 and NaHCO3 were

12

deemed as the best draw solutes, while KHCO3, MgSO4, NaCl, NaHCO3 and Na2SO4 were found

to be the most cost-effective in terms of replenishment costs.

Various synthetic draw solutes have been synthesized. For instance, polyelectrolytes of various

polyacrylic acid sodium salts (PAA-Na) have been proposed as possible draw solutes in FO. Due

to their high solubility, they are capable of providing water fluxes similar to conventional ionic

salts with significantly lower backflow. Furthermore, they can be easily recycled by

ultrafiltration. (Ge, Su, Amy & Chung, 2012)

Highly water-soluble magnetic nanoparticles, functionalized by polyacrylic acid, were found to

produce high osmotic pressure and water flux in FO. Key parameters to their efficiency include

their surface hydrophilicity and particle size. Although the magnetic nanoparticles can be easily

recovered from the permeate using a magnetic field, agglomeration of the particles occurs. While

ultrasonication is capable of reducing the agglomeration, the process weakens the resultant

magnetic properties of the particles. (Ling, Wang & Chung, 2010). The use of hydrogels as a

draw agent in FO has also been investigated. Due to their hydrophilic and flexible polymeric

network, hydrogels may be used to absorb water through a semi-permeable membrane. The

swollen hydrogels can then be reversibly dewatered by temperature, pressure, or solar

irradiation. (Li, Zhang, Simon & Wang, 2013). However, the water fluxes observed were lower

than those produced by conventional draw solutes, and the high cost of dewatering also hinders

its practical applications.

Recently, an integrated FO–UF (forward osmosis–ultrafiltration) system employing super

hydrophilic nanoparticles as draw solutes was developed. The nanoparticles are used to draw

water across the membrane, and are subsequently regenerated by UF membranes. (Ling &

Chung, 2011.)

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1.4.5 Operating temperature

Increasing the temperature of the draw and/or feed solutions increases water flux. (Garcia-

Castello, McCutcheon & Elimelech, 2009; Zhao & Zou, 2011). This can be attributed to an

increase in solute diffusion coefficients and a decrease in solution viscosity. In a bench-scale

experiment where FO was used for the desalination of brackish water, it was determined that the

use of higher temperatures (between 25°C and 45 °C) increased initial water flux, total water

recovery and concentration factors. However, membrane scaling was aggravated and more

frequent cleaning was required. (Zhao & Zou, 2011).

The use of higher temperatures has also been found to reduce reverse solute diffusion and

increase salt rejection rates. Phuntso et al (2012) found that increasing the temperature of the

draw solution alone significantly decreased ICP. Interestingly, increasing the temperature of the

draw solution only (25°C to 45°C) resulted in a greater increase in flux, as compared to when the

temperature of the feed solution was increased.

1.4.6 Draw solution concentration

The concentration factor that can be attained in FO is dependent on the osmotic pressure of the

draw solution, which affects the osmotic gradient across the membrane. A higher draw solution

concentration will yield higher transmembrane flux rates and greater salt rejection. In one study,

an increase in the concentration of NaCl in the draw solution from 6% to 26% (w/w) caused an

increase in flux from 0.44 L/m2h-1 to 1.39 L/m2h-1. When the concentration of sucrose in the

draw solution was increased from 30% to 50% (w/w) caused an increase in flux from 0.28

L/m2h-1 to 0.58 L/m2h-1. (Raghavarao, Madhusudhan, Hrishikesh Tavanandi & Niranjan, 2014).

In another sucrose concentration study by Petrotos, Quantick and Petropakis (1998), flux

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increased linearly with draw solution concentration, but at a much slower rate. A mere 30%

increase in osmotic flux was observed when the draw solution concentration was increased by

360%, due to increased mass transfer resistance.

1.4.7. Flow rate

Increasing the flow rate of the feed or/and draw solutions enables a greater flux to be attained.

This is because a faster flow rate decreases the thickness of the hydrodynamic boundary layer

and increases the Reynolds number, hence reducing the mass transfer resistance of the

concentration polarization layer next to the membrane. (Sant’Anna, Marczak & Tessaro, 2012).

However, Petrotos et al. (1998) reported only a small increase (32%) in flux when faster flow

rate was used (109 l/h up to 502 l/h), possibly due to the high viscosity of the tomato juice feed

solution. Increased feed flow rate also reduces membrane fouling due to the increased shear

forces generated. (Lee, Elimenech & Hong, 2010).

1.5. Challenges

Aside from concentration polarization, which has been discussed above, FO is limited by

membrane fouling and reverse solute diffusion, which will be discussed below.

1.5.1 Membrane fouling

Fouling is a significant problem in virtually all pressure-driven membrane processes, and

demands frequent maintenance of the membranes to maintain process efficiency. Fouling control

strategies include the pretreatment of feed water, designing membranes that are more resistant

fouling, and by modifying operating conditions. However, physically irreversible fouling cannot

15

be removed by physical cleaning or pretreatment. Hence, chemical cleaning is usually used,

despite its negative impact on membrane life and membrane selectivity.

When compared to pressure-driven membrane processes, FO offers the advantage of being less

susceptible to fouling due to reduced cake layer compaction. This is due to the lower hydraulic

pressures involved in the process. Organic fouling of FO membranes was also shown to have

greater reversibility, reducing or even possibly eliminating the need for chemical cleaning.

Physical cleaning methods such as hydraulic cleaning may be sufficient for fouling control of FO

membranes. In study by Achilli, Cath, Marchand & Childress (2009), in which an OMBR was

used to treat domestic wastewater, a decline in water flux decline was observed and attributed to

membrane fouling. However, the use of osmotic backwashing on a weekly basis was able to

restore the flux to 90% of its initial value. In this process, the draw solution was replaced with

deionized water, reversing the osmotic pressure gradient. The flow of water across the membrane

into the feed solution helps to dissolve and remove fouling deposits from the surface of the

membrane. (Coday et al. 2014). In another study on organic fouling of FO membranes, Lee, Boo,

Elimenech & Hong (2010) found that an almost complete recovery in flux could be achieved by

increasing the cross-flow velocity. When the same conditions were applied to an RO system, no

recovery in flux was observed.

When organic fouling occurs in salt-rejecting membranes, the drop in transmembrane flux is

determined by the increase in the total hydraulic resistance caused by the cake layer. This

increase in hydraulic resistance is in turn controlled by the compactness and thickness of the

organic fouling layer. (Hong & Elimenech, 1997). Cake-enhanced osmotic pressure (CEOP) is a

critical mechanism responsible for flux decline in salt-rejecting membranes, in which colloidal

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deposit layers hinder the back-diffusion of salt ions, causing increased osmotic pressure at the

membrane surface.

Lee, Boo, Elimenech & Hong (2010) found that the effect of fouling on flux depends on the

nature of the organic fouling agent, the size of the colloidal fouling agent, and the type of the

draw solution employed. In a study by Mi and Elimenech (2008), it was shown that organic

fouling is dependent on the combination of chemical and hydrodynamic interactions. Major

factors affecting membrane fouling include calcium binding of organic macromolecules,

permeation drag, and hydrodynamic shear force. Stronger intermolecular adhesion forces

promote deposition and cake formation.

Membrane fouling can be reduced by using a conventional FO membrane configuration, in

which the feed solution faces the active layer. Stable flux can be achieved using this

configuration, although more severe ICP occurs in the support layer. A higher initial flux can be

achieved using the alternate configuration, but internal clogging occurs in the support layer with

time, increasing ICP and causing a sharp decrease in flux. (Tang, She, Lay, Wang & Fe, 2010)

1.5.2 Reverse solute flux

Backflow of solutes is a result of concentration polarization phenomena. It results in a reduction

of water flux, loss of draw solute, and contamination of the feed solution. In commercially

available cellulose acetate FO membranes, reverse salt flux was estimated to range between 80

mg to nearly 3,000 mg per liter of water produced.

While FO processes should be operated with low feed and draw solution flow velocities to

minimize backflow, this comes at the cost of overall process performance: increased ECP will

exacerbate membrane fouling and reduce water flux. (Hancock & Cath, 2009). Operating at a

17

high draw solution flow velocity with a low feed flow velocity has been found to cause the most

draw solute loss.

Divalent feed solutes exhibit lower permeation rates of less than 1 mmol/m2 per hours, while

monovalent ions and uncharged solutes are more prone to backflow. However, the larger ionic

size and lower solution diffusion coefficients of multivalent solutes may worsen ICP.

Multivalent draw solutes may reduce membrane fouling as they are less prone to backflow. On

the contrary, the reverse diffusion of some multivalent ions like calcium and magnesium ions

may worsen membrane fouling by interacting with the fouling agents.

It was determined that specific reverse solute flux is dependent on the selectivity of the FO

membrane, and is independent of the structure of the membrane support layer and the

concentration of the bulk draw solution. To minimize this phenomenon, an FO membrane with a

highly selective active layer should be developed. (Phillip, Yong & Elimenlech, 2010)

1.6. Food applications of forward osmosis

1.6.1 Concentration of fruit and vegetable juices

The first use of FO for the concentration of fruit juice was reported by Popper, Camirand, Nury

& Stanley (1966). The process was used to concentrate grape juice from 16 to 60 °Brix, using

tubular and flat sheet RO cellulose acetate membranes and brine as the draw solution. However,

high reverse salt flux occurred, negatively impacting the sensory attributes of the juice. In 1990,

Beaudry and Lampi successfully concentrated orange juice to 42 °Brix using a 72 °Brix sugar

syrup as the draw solution. The refrigeration of the feed solution enabled favorable flavor, color,

and sensory characteristics to be retained.

Wrolstad et al. (1993) concentrated red raspberry juice from 10 to 45 Brix using vacuum

18

evaporation as well as FO, using high-fructose corn syrup as the draw solution. It was observed

that both concentration processes lead to minor losses of anthocyanin as well as slight increases

in percent polymeric color, which is an indicator of the degree of anthocyanin degradation.

Sensory analysis found no significant differences between both concentrates, single-strength

juice, and three of the commercial samples tested. However, in comparison to the concentrates

obtained using vacuum evaporation, the FO concentrates were more similar to single-strength

juice in terms of red raspberry flavor and aroma.

Beaudry, Jochums & Medina (1994) used FO for the concentration of tomato juice from 6.0 to

75.9 Brix, using high fructose corn syrup as the osmotic agent. The same process was also used

to concentrate coffee from 5 to between 56 and 63 Brix. Orange juice was concentrated from

12.6 to about 50 Brix using polyethylene glycol as the draw solution. In 2010, Petrotos et al.

developed a novel membrane module for tomato juice concentration by FO. Using brine as the

draw solution, fresh tomato juice was concentrated from 5.5° to 15.88 °Brix, and from 4.25 to

7.5 °Brix. The concentration factors obtained were sufficient to meet standards for passata and

pizza sauce, which should range between 7–9 and 10–14°Brix respectively. The use of

electrodialysis for the regeneration of the diluted draw solution was proposed, as it is more cost-

effective than evaporization. In a previous FO study, Petrotos, Quantick and Petropakis (1999)

determined that pre-treating tomato juice with filtration, microfiltration, ultrafiltration increased

the osmotic flux.

Nayak, Valluri and Rastogi (2011) investigated the concentration of beetroot, grape, and

pineapple juices using FO. Using a NaCl draw solution, °Brix levels for each of the juice types

increased from 2.3 to 52 °Brix, from 8.0 to 54.6 °Brix, and from 4.4 to 54 °Brix respectively.

19

The betalain content in beetroot juice increased from 50.92 mg/L to 2.91 g/L, while the

anthocyanin content in grape juice increased from 104.85 mg/L to 715.6 mg/L.

In a sucrose concentration experiment by Garcia-Castello, McCutcheon & Elimelech (2012), a

concentration factor of 5.7 was achieved using FO. This was more than twice than the

concentration factors attained with RO reported in scientific literature, which were capped at 2.5.

The reverse diffusion of draw solutes to the feed solution limits the use of FO in food

applications. NaCl is commonly used as a draw solute in FO, but reverse salt migration has a

negative impact on the organoleptic and nutritional quality of the feed solution. Sucrose has also

been used as a draw solute in FO, but yields a low flux. To mitigate the disadvantages of both

draw solutes, Babu, Rastogi & Raghavarao (2006) used a mixture of NaCl and sucrose as the FO

draw solution in the concentration of pineapple juice. When a draw solution comprising 30%

(w/w) sucrose and a variable amount of NaCl was used, the transmembrane flux increased from

0.28 to 1.13 l/m2 h as the NaCl concentration was increased from 0% to 16% (w/w). However,

sodium chloride transfer to the feed solution increased with the concentration of NaCl in the

draw solution. When the highest NaCl concentration was used, the feed solution was found to

contain 1.28% NaCl. When a draw solution comprising 12% and a variable amount of sucrose

was used for the FO process, the transmembrane flux increased from 0.89 L/m2h to 1.18 L/m2h

as the sucrose concentration was increased from 0% to 40%.The addition of sucrose also reduced

NaCl migration from 1.87% to 0.58%. This may be due to the increase in the viscosity of the

draw solution as the sucrose concentration was increased. A maximum Brix value of 60 was

achieved with the use of a draw solution containing 12% NaCl and 40% sucrose. This draw

solution also achieved the best sensory evaluation scores with regards to saltiness, sweetness and

20

overall acceptability. Using this draw solution, it was observed that the transmembrane flux

increased by 78% when the feed temperature was increased from 25◦C to 45◦C. Furthermore, it

was noted that osmotic flux increases when the Reynolds number (flow rate) of the feed or draw

solution was increased.

1.6.2 Concentration of byproducts from food processing

FO can be used as a pretreatment step in the disposal or recovery of waste. Aydiner et al. (2012)

investigated the effectiveness of an integrated membrane system which utilized FO for whey

concentration and RO for the recovery of water from the draw solution. FO concentration of

whey was performed for 6 hours, increasing its solid content from 6.8 to 14.3%. The process did

not attain steady-state at the end of the 6 hours, and the authors estimated that further

concentration of the whey to a solid content of 25% to 35% of solid content could be achieved

with a longer process time of 15 to 20 hours. While the performance of the process was

satisfactory, high salt transfer from the draw solution into the whey was observed. Some soluble

organic compounds from the whey were also found in the draw solution. As such, the authors

inferred that the NaCl concentration of the FO draw solution should not exceed 3M to avoid high

salt permeation into the whey. While pretreatment with microfiltration can be used to recover

fats from the whey, it reduced the performance of the FO process. It was concluded that the

system investigated could be used as an alternative to the ultrafiltration-RO system widely used

for the concentration of whey, but further optimization of the system would be required.

Pal and Nayak (2016) developed a membrane-integrated hybrid reactor system for the production

of whey protein and acetic acid from sweet whey. FO was used for the concentration of the

acetic acid solution, attaining a concentration of 962 g/L. FO was also used for the pre-

21

concentration of whey, which was estimated to have an initial protein content of the whey was

estimated to be 0.6–0.65%. The process yielded a concentrate containing 954.5g protein/L,

which was subsequently vacuum dried to obtain a powder.

Orange peel press liquor is a byproduct from orange juice production, which is usually

concentrated to 72 Brix to produce high-value citrus molasses. The use of RO as a

preconcentration step was previously investigated and achieved a low maximum concentration of

11◦ Brix. (Garcia-Castello & McCutcheon, 2011). A similar concentration factor was achieved

with the use of FO, yielding a concentrate of 11.13 Brix (concentration factor of 1.39) with the

use of a 2M NaCl draw solution. The use of a 4M NaCl draw solution did not improve the

dewatering process, and yielded a concentrate of 11.13 Brix (concentration factor of 1.32).

Pectin was determined to be the main cause of fouling, and a much higher concentration factor

was achieved when a synthetic press liquor without pectin was used. With the use of 2M NaCl

and 4M NaCl draw solutions, Brix values of 18.71 (concentration factor of 2.17) and 31.68

(concentration factor of 3.67) were obtained respectively.

1.6.3 Concentration of anthocyanin extracts

When thermal evaporation is used to concentrate natural colorants, color degradation may occur.

It is desirable to use non-thermal techniques or techniques that apply less heat treatment to

improve the shelf life and stability of natural color extracts. Various membrane processes,

including ultrafiltration and RO, have been used for the clarification and concentration of these

extracts.

Rodriguez-Saona, Giusti, Durst and Wrolstad (2001) concentrated red radish juice using FO and

thermal evaporation, obtaining concentrates of 5.1 and 15.5 Brix respectively. The concentrates

22

contained 55.2 mg/L and 170.3 mg/L of monomeric anthocynanin. The processes required 8

hours and 2 hours respectively. A juice extract with a Brix value of 8.2 and a monomeric

anthocyanin concentration of 109.8 mg/L was obtained when two rounds of FO was performed

in ten hours. While the FO process required significantly more time, the juice developed an off-

flavor when subjected to temperatures exceeding 45 in the thermal evaporator. The combination

of FO and thermal evaporation presents a promising alternative concentration technique. The

process produced a concentrate with a high Brix value (11.8) and anthocyanin concentration

was obtained in merely 3 hours. Using sensory evaluation, the extract obtained from the former

process was determined to have a lower aroma intensity than the thermally evaporated sample.

Both extracts obtained by thermal evaporation and the combined process were determined to

produce the desired color of FDSC Red #40.

Nayak and Rastogi (2012) used FO and osmotic distillation to concentrate anthocyanins from a

crude extract of kokum (Garcinia indica). FO achieved a higher concentration factor and

transmembrane flux (49.63 mg/L to 2.69 g/Lin 18 hours) than osmotic membrane distillation

(72mg/L in 18 hours). However, NaCl migration from the draw solution (0.21 moles/m2s) into

the feed solution was observed during FO, while no transfer of osmotic agent occurred when the

latter process was used. When the NaCl concentration was raised from 1.0 to 6.0 M, FO and

osmotic distillation transmembrane flux increased from 0.14 to 0.68L/m2h and from 7.5 to

12.3L/m2h respectively. Furthermore, the anthocyanin extracts obtained using FO and osmotic

distillation exhibited higher stability than the extract produced by thermal degradation; the

degradation constant of the thermally concentrated sample (63.0 x 10-3 day-1) was eight times

higher than that of the FO extract (8.0 x 10-3 day-1). Furthermore, less osmotic browning

occurred when FO was used; the non- enzymatic browning index for the fresh kokum sample,

23

FO extract, thermally concentrated extract was determined to be 0.25, 0.35 and 0.78 respectively.

Less conversion of hydroxycitric acid (HCA) lactone to HCA occurred when FO was used

(1.50:1), as compared to when thermal concentration was employed (2.84:1).

In another study by Jampani and Raghavarao (2015), red cabbage extract was concentrated using

thermal evaporation, a combination of ultrafitration and FO, and a combination of multistage

aqueous two phase extraction (ATPE) followed by FO or osmotic distillation. The combination

of ATPE and FO was found to yield the highest concentration of anthocyanins (3123.45 mg/L

and 43 °Brix) or a 14.1 fold concentration. The same process also produced the lowest non-

enzymatic browning index (0.11) and degradation constant (0.21), as well as the highest color

density (14.56) and polymeric color (12.56%).

In a similar experiment on jamun, the combination of ATPE and FO was also found to yield the

highest concentration of anthocyanins (2890.32 mg/L and 40 °Brix). (Chandrasekhar &

Raghavarao, 2015). The same process also yielded the lowest non-enzymatic browning index

(0.15), the highest color density (12.36), polymeric color (11.89%), density (1167kg/m3). A low

degradation constant was observed (0.24 day-1), but the combination of ATPE and osmotic

distillation produced an extract with a slightly lower degradation constant (0.21 day-1). In both

experiments, both combined ATPE membrane concentration processes (ATPE & FO and ATPE

& osmotic distillation) yielded high anthocyanin stability in relation to pH and temperature.

24

1.7. References

Achilli, A., Cath, T. Y., & Childress, A. E. (2010). Selection of inorganic-based draw solutions

for forward osmosis applications. Journal of Membrane Science, 364(1), 233-241.

Achilli, A., Cath, T. Y., Marchand, E. A., & Childress, A. E. (2009). The forward osmosis

membrane bioreactor: a low fouling alternative to MBR processes. Desalination, 239(1), 10-21.

Aydiner, C., Topcu, S., Tortop, C., Kuvvet, F., Ekinci, D., Dizge, N., & Keskinler, B. (2013). A

novel implementation of water recovery from whey:“forward–reverse osmosis” integrated

membrane system. Desalination and Water Treatment, 51, 786-799.

Babu, B. R., Rastogi, N. K., & Raghavarao, K. S. M. S. (2006). Effect of process parameters on

transmembrane flux during direct osmosis. Journal of membrane science, 280(1), 185-194.

Beaudry, E. G., & Lampi, K. A. (1990). Osmotic concentration of fruit juices. Fluessiges

Obstretics, 57(10), 662-656, 663-664.

Cath, T. Y., Childress, A. E., & Elimelech, M. (2006). Forward osmosis: principles, applications,

and recent developments. Journal of membrane science, 281(1), 70-87.

Chandrasekhar, J., & Raghavarao, K. S. M. S. (2015). Separation and concentration of

anthocyanins from jamun: an integrated process. Chemical Engineering Communications,

202(10), 1368-79.

Chung, T. S., Zhang, S., Wang, K. Y., Su, J., & Ling, M. M. (2012). Forward osmosis processes:

yesterday, today and tomorrow. Desalination, 287, 78-81.

25

Coday, B. D., Xu, P., Beaudry, E. G., Herron, J., Lampi, K., Hancock, N. T., & Cath, T. Y.

(2014). The sweet spot of forward osmosis: treatment of produced water, drilling wastewater,

and other complex and difficult liquid streams. Desalination, 333(1), 23-35.

Cornelissen, E. R., Harmsen, D., De Korte, K. F., Ruiken, C. J., Qin, J. J., Oo, H., & Wessels, L.

P. (2008). Membrane fouling and process performance of forward osmosis membranes on

activated sludge. Journal of Membrane Science, 319(1), 158-168.

Fang, Y., Bian, L., Bi, Q., Li, Q., & Wang, X. (2014). Evaluation of the pore size distribution of

a forward osmosis membrane in three different ways. Journal of Membrane Science, 454, 390-

397.

Garcia-Castello, E. M., & McCutcheon, J. R. (2011). Dewatering press liquor derived from

orange production by forward osmosis. Journal of Membrane Science, 372(1), 97-101.

Garcia-Castello, E. M., McCutcheon, J. R., & Elimelech, M. (2009). Performance evaluation of

sucrose concentration using forward osmosis.Journal of membrane science, 338(1), 61-66.

Ge, Q., Su, J., Amy, G. L., & Chung, T. S. (2012). Exploration of polyelectrolytes as draw

solutes in forward osmosis processes. Water research, 46(4), 1318-1326.

Ge, Q., Su, J., Chung, T. S., & Amy, G. (2010). Hydrophilic superparamagnetic nanoparticles:

synthesis, characterization, and performance in forward osmosis processes. Industrial &

Engineering Chemistry Research, 50(1), 382-388.

Hancock, N. T., & Cath, T. Y. (2009). Solute coupled diffusion in osmotically driven membrane

processes. Environmental science & technology, 43(17), 6769-6775.

26

Herron, J. (2008). U.S. Patent No. 7,445,712. Washington, DC: U.S. Patent and Trademark

Office.

Herron, J. R., Beaudry, E. G., Jochums, C. E., & Medina, L. E. (1994). U.S. Patent No.

5,281,430. Washington, DC: U.S. Patent and Trademark Office.

Hong, S., & Elimelech, M. (1997). Chemical and physical aspects of natural organic matter

(NOM) fouling of nanofiltration membranes. Journal of membrane science, 132(2), 159-181.

Jampani, C., & Raghavarao, K. S. M. S. (2015). Process integration for purification and

concentration of red cabbage (Brassica oleracea L.) anthocyanins. Separation and Purification

Technology, 141, 10-16.

Lee, S., Boo, C., Elimelech, M., & Hong, S. (2010). Comparison of fouling behavior in forward

osmosis (FO) and reverse osmosis (RO). Journal of Membrane Science, 365(1), 34-39.

Li, D., Zhang, X., Simon, G. P., & Wang, H. (2013). Forward osmosis desalination using

polymer hydrogels as a draw agent: Influence of draw agent, feed solution and membrane on

process performance. Water research, 47(1), 209-215.

Ling, M. M., & Chung, T. S. (2011). Desalination process using super hydrophilic nanoparticles

via forward osmosis integrated with ultrafiltration regeneration. Desalination, 278(1), 194-202.

Ling, M. M., Wang, K. Y., & Chung, T. S. (2010). Highly water-soluble magnetic nanoparticles

as novel draw solutes in forward osmosis for water reuse. Industrial & Engineering Chemistry

Research, 49(12), 5869-5876.

Mallevialle, J., Odendaal, P. E., & Wiesner, M. R. (Eds.). (1996). Water treatment membrane

processes. American Water Works Association.

27

McCutcheon, J. R., McGinnis, R. L., & Elimelech, M. (2006a). Desalination by ammonia–

carbon dioxide forward osmosis: influence of draw and feed solution concentrations on process

performance. Journal of membrane science, 278(1), 114-123.

McCutcheon, J. R., & Elimelech, M. (2006b). Influence of concentrative and dilutive internal

concentration polarization on flux behavior in forward osmosis. Journal of Membrane Science,

284(1), 237-247.

McCutcheon, J. R., McGinnis, R. L., & Elimelech, M. (2005). A novel ammonia—carbon

dioxide forward (direct) osmosis desalination process.Desalination, 174(1), 1-11.

Mehta, G. D. (1982). Further results on the performance of present-day osmotic membranes in

various osmotic regions. Journal of Membrane Science, 10, 3–19.

Mi, B., & Elimelech, M. (2008). Chemical and physical aspects of organic fouling of forward

osmosis membranes. Journal of Membrane Science, 320(1), 292-302.

Nayak, C. A., & Rastogi, N. K. (2010). Comparison of osmotic membrane distillation and

forward osmosis membrane processes for concentration of anthocyanin. Desalination and Water

Treatment, 16(1-3), 134-145.

Nayak, C. A., Valluri, S. S., & Rastogi, N. K. (2011). Effect of high or low molecular weight of

components of feed on transmembrane flux during forward osmosis. Journal of Food

Engineering, 106(1), 48-52.

Nicoll, P. (2013). Forward osmosis - a brief introduction. Paper presented at The International

Desalination Association World Congress on Desalination and Water Reuse: Tianjin, China.

28

Pal, P., & Nayak, J. (2016). Development and analysis of a sustainable technology in

manufacturing acetic acid and whey protein from waste cheese whey. Journal of Cleaner

Production, 112, 59-70.

Petrotos, K. B., Quantick, P. C., & Petropakis, H. (1999). Direct osmotic concentration of tomato

juice in tubular membrane–module configuration. II. The effect of using clarified tomato juice on

the process performance. Journal of membrane science, 160(2), 171-177.

Petrotos, K. B., Quantick, P., & Petropakis, H. (1998). A study of the direct osmotic

concentration of tomato juice in tubular membrane–module configuration. I. The effect of certain

basic process parameters on the process performance. Journal of Membrane Science, 150(1), 99-

110.

Petrotos, K. B., Tsiadi, A. V., Poirazis, E., Papadopoulos, D., Petropakis, H., & Gkoutsidis, P.

(2010). A description of a flat geometry direct osmotic concentrator to concentrate tomato juice

at ambient temperature and low pressure. Journal of food engineering, 97(2), 235-242.

Phillip, W. A., Yong, J. S., & Elimelech, M. (2010). Reverse draw solute permeation in forward

osmosis: modeling and experiments. Environmental science & technology, 44(13), 5170-5176.

Phuntsho, S., Vigneswaran, S., Kandasamy, J., Hong, S., Lee, S., & Shon, H. K. (2012).

Influence of temperature and temperature difference in the performance of forward osmosis

desalination process. Journal of membrane science, 415, 734-744.

Popper, K., Camirand, W. M., Nury, F., & Stanley, W. L. (1966). Dialyzer concentrates

beverages. Journal of Food Engineering, 38(4), 102-104.

29

Raghavarao, K. S. M. S., Madhusudhan, M. C., Hrishikesh Tavanandi, A., Niranjan, K. (2014).

Athermal Membrane Processes for the Concentration of Liquid Foods and Natural Colours. In D.

W. Sun (Eds.), Emerging Technologies for Food Processing, (pp. 213-238). London, England:

Academic Press.

Rodriguez-Saona, L. E., Giusti, M. M., Durst, R. W., & Wrolstad, R. E. (2001). Development

and process optimization of red radish concentrate extract as potential natural red

colorant. Journal of food processing and preservation, 25(3), 165-182.

Sant’Anna, V., Marczak, L. D. F., & Tessaro, I. C. (2012). Membrane concentration of liquid

foods by forward osmosis: process and quality view. Journal of food engineering, 111(3), 483-

489.

Tang, C. Y., She, Q., Lay, W. C., Wang, R., & Fane, A. G. (2010). Coupled effects of internal

concentration polarization and fouling on flux behavior of forward osmosis membranes during

humic acid filtration. Journal of Membrane Science, 354(1), 123-133.

Tang, C. Y., She, Q., Lay, W. C., Wang, R., Field, R., & Fane, A. G. (2011). Modeling double-

skinned FO membranes. Desalination, 283, 178-186.

Thompson, N. A., & Nicoll, P. G. (2011, September). Forward osmosis desalination: a

commercial reality. In IDA World Congress–Perth Convention and Exhibition Centre (PCEC),

Perth, Western Australia September (pp. 4-9).

Wang, K. Y., Ong, R. C., & Chung, T. S. (2010). Double-skinned forward osmosis membranes

for reducing internal concentration polarization within the porous sublayer. Industrial &

Engineering Chemistry Research, 49(10), 4824-4831.

30

Wang, R., Shi, L., Tang, C. Y., Chou, S., Qiu, C., & Fane, A. G. (2010). Characterization of

novel forward osmosis hollow fiber membranes. Journal of membrane science, 355(1), 158-167.

Wei, J., Liu, X., Qiu, C., Wang, R., & Tang, C. Y. (2011). Influence of monomer concentrations

on the performance of polyamide-based thin film composite forward osmosis membranes.

Journal of Membrane Science, 381(1), 110-117.

When-What-How. Nanofiltration Separations Part 1 (Nanotechnology). http://what-when-

how.com/nanoscience-and-nanotechnology/nanofiltration-separations-part-1-nanotechnology/

Wrolstad, R. E., McDaniel, M. R., Durst, R. W., Micheals, N., Lampi, K. A., & Beaudry, E. G.

(1993). Composition and Sensory Characterization of Red Raspberry Juice Concentrated by

Direct‐Osmosis or Evaporation. Journal of Food Science, 58(3), 633-637.

Xu, Y., Peng, X., Tang, C. Y., Fu, Q. S., & Nie, S. (2010). Effect of draw solution concentration

and operating conditions on forward osmosis and pressure retarded osmosis performance in a

spiral wound module. Journal of Membrane Science, 348(1), 298-309.

Zhao, S., & Zou, L. (2011). Effects of working temperature on separation performance,

membrane scaling and cleaning in forward osmosis desalination. Desalination, 278(1), 157-164.

Zhao, S., Zou, L., Tang, C. Y., & Mulcahy, D. (2012). Recent developments in forward osmosis:

opportunities and challenges. Journal of Membrane Science, 396, 1-21.

31

CHAPTER 2

FORWARD OSMOSIS AS A NONTHERMAL METHOD

OF ACID WHEY CONCENTRATION

2.1. Introduction

Acid whey is a byproduct obtained from strained yogurt and acid-coagulated cheese

production. The recent popularity of Greek yogurt has led to a large increase in the production of

this byproduct. About 500,000 tons of Greek yogurt are produced per year in the US, which

equates to 1 million tons of acid whey. (Arla, 2014.) Due to its high organic matter content and

the large volumes produced, the disposal of acid whey poses a significant problem for the dairy

industry.

Due to its potential negative effect on the environment, acid whey cannot be disposed

without prior treatment. Current methods of disposing whey include animal feeding, land

application as a fertilizer, anaerobic digestion, and treatment in wastewater plants. (Ling, 2008;

NYSDEC, n.d.) However, acid whey contains lactose, protein, minerals, and lactic acid, which

could be utilized in the production of various chemicals and food products. Acid whey can also

be concentrated to reduce storage and transportation costs, and to improve its shelf-life and

stability. Membrane technologies enable food products to be concentrated non-thermally,

preventing protein denaturation and changes in sensory properties that occur when thermal

evaporation is used. Typically, whey is concentrated using reverse osmosis (RO) or thermal

evaporation, and then dried into a powder (Tetra Pak. 1995).

32

Forward Osmosis (FO) represents a promising method for concentrating liquid foods

without requiring the application of heat or hydraulic pressure, hence enabling the retention of

functional and sensory properties. The process is exclusively driven by the osmotic pressure

gradient between two solutions separated by a semi-permeable membrane. FO requires a highly

concentrated draw solution to produce a high osmotic pressure that spontaneously drives water

flow from the feed solution into the draw solution. As a result, the feed solution is concentrated

and the draw solution is diluted.

To date, there has been limited use of FO in the food industry. Compared to hydraulic

pressure-driven membrane technologies like RO, FO equipment tends to be simpler in design,

membranes require less membrane support and have a lower propensity for fouling, since lower

pressures are involved. (Cath, Childress, & Elimelech, 2006.) As the primary energy requirement

in FO is for the regeneration of the draw solution, the overall energy cost of the process can be

significantly lower than that pressure-driven membrane processes if the draw solution can be

easily recovered or discarded. Furthermore, FO has the potential to achieve a higher

concentration factors than RO, as the osmotic pressure gradients in the former are much greater

than the hydraulic driving forces in RO. (Garcia-Castello, McCutcheon & Elimelech, 2009.)

The goal of this research is to evaluate FO as a method for concentrating acid whey from

skim Greek yogurt production. The results from this study will provide flux and concentration

data, which will help us evaluate the feasibility of implementing this technology in the dairy

industry.

33

2.2. Materials and Methods

Feed solution

Skim Greek yogurt acid whey was obtained from Byrne Dairy (Cortland, NY). All acid

whey samples were refrigerated at 8 ºC, and used within 6 days. Prior to FO concentration, the

acid whey was filtered using Whatman Grade 41 ashless filter paper to remove fines. The

composition of acid whey was determined by Medallion Labs (Minneapolis, MN).

Two sets of experiments were performed. In the first set, 12L of raw acid whey (6.2 ± 0.1

ºBrix) was concentrated using FO for 165 min. The concentrate obtained was used for the second

set of experiments, in which 1.8L of pre-concentrated acid whey (28.8 ± 0.7 ºBrix) was

concentrated for 35 min. The products from the first and second experiments will be termed

Concentrate I and II respectively.

Draw solution

The FO draw solution was provided by Ederna (Toulouse, France). The draw solution

was stored at ambient temperature, and its ºBrix was measured before use. If the concentration of

the draw solution was above 60.0 ºBrix, it was diluted to 59.5- 60.0 ºBrix using distilled water.

The spent draw solution was concentrated to 60-70 ºBrix using vacuum evaporation at a

temperature ranging between 82.2 and 87.8 ºC. Before use, the regenerated osmotic agent was

heated to 70 ºC and filtered using Whatman Grade 41 ashless filter paper to remove any

particulates that could damage the membrane. It was then diluted to 59.5-60 ºBrix using distilled

water.

Forward osmosis system

The experiments were conducted using a FO lab unit (Ederna, Toulouse, France)

illustrated in Figure 2.1. The FO system comprised a feed solution vessel connected to a

34

variable-speed centrifugal pump, a draw solution vessel connected to a variable-speed centrifugal

pump, a plate heat exchanger connected to a recirculating cooler, and a spiral-wound cellulose

membrane with an effective area of 0.50 m2, placed inside stainless steel housing. The unit was

also equipped with a temperature probe and analog pressure gauges.

Figure 2.1. Schematic diagram of forward osmosis system. Adapted from Ederna (2016).

The feed solution was circulated in a closed loop, and the draw solution was circulated in

an open loop. In the raw acid when concentration experiments, a 20 L polypropylene carboy was

used to contain the feed solution. For the concentration of pre-concentrated acid whey, a 1L

stainless steel feed solution flask (Ederna, Toulouse, France) was used for the feed solution. The

flow rate of the draw solution loop was approximately 196 mL/min. Using deionized water as the

feed solution, the flow rate of the feed solution loop was determined to range between 3.9 L/min

and 4.2 L/min, depending on the opening of the valve.

35

A plate heat exchanger and cooling circulator were used to maintain the feed solution at a

cold temperature throughout the experiment, to avoid both microbial growth during processing

and any changes in product quality. The temperature of the cooler was preset at the beginning of

the FO runs to 5 ºC for raw acid whey and to 7 ± 2.8ºC for pre-concentrated whey runs. Due to

technical limitations, the cooler was unable to maintain the temperature of the feed solution

during the course of the experiments.

Raw acid whey concentration experiments were performed in quadruplicate, and pre-

concentrated acid whey concentration experiments were performed in duplicate.

Membrane cleaning

After each FO experiment, chemical cleaning was performed. Deionized water was

circulated through the osmotic agent loop until a ºBrix value below 1.0 was obtained. The feed

solution loop was rinsed with deionized water until the effluent was clear, and subjected to

alkaline cleaning with Ultrasil 110 (Ecolab, St. Paul, MN) of concentration of 1% (v/v), which

was carried out for 15 min at 25 ºC. The membrane was then rinsed with deionized water until a

neutral pH was attained. After that, the membrane was cleaned with TergazymeTM enzyme-active

powdered detergent (Alconox, White Plains, NY) at a concentration of 1% (w/v), for 15 min at

40ºC. Finally, the membrane was cleaned with pH 4 citric acid solution at 25 ºC for 15 min.

Between experimental runs, the membrane was stored in pH 4 citric acid solution.

Monitoring membrane performance

The membrane performance was monitored by determining the water flux before each

FO run. This was determined by measuring the time required for 1L of deionized water to pass

through the membrane, from the feed solution to the draw solution.

36

Flux determination

The transmembrane flux was determined gravimetrically, based on the weight change of

the draw solution. The water flux through the membrane (J) was calculated as:

∗ ∗ (2.1)

where J: flux (L/m2h)

: increase in weight (kg) of the spent DS during the time interval t (h)

: decrease in weight (kg) of the DS during the time interval t

A: area of the membrane (m2)

: density of acid whey (kg/m3)

For the concentration of raw acid whey and pre-concentrated acid whey, the initial flux

was taken 20 min and 7.5 min after starting the pump respectively, after the system was fully

stabilized. After that, flux measurements were performed at 10 min intervals for the

concentration of raw acid whey, and at 5 min intervals for the concentration of pre-concentrated

acid whey.

Physicochemical Analyses

The concentration of the feed solution and feed temperature was measured at the same

time intervals as the flux, using a Sper Scientific Pocket Digital Refractometer (Scottsdale, AZ),

and reported in ºBrix.

The total solids content was measured using the AOAC Method 925.23.

Estimation of total solids content from ºBrix. A direct conversion between ºBrix and

total solids content was determined by building a calibration curve of ºBrix versus total solids

content, within a ºBrix range of 7.1 and 44.6.

37

Conductivity was determined using the Fisher Scientific Traceable Conductivity,

Resistivity, and TDS Meter (Waltham, MA).

Water activity was determined using the AquaLab Dew Point Water Activity Meter 4TE

(Ramsey, NJ). The pH of the acid whey and concentrated product were measured using a Fisher

Scientific Accumet Excel XL20 pH meter (Fisher Scientific, Pittsburgh, PA) at 20ºC.

The Lab color parameters were determined using the Konica Minolta CR-400 Chroma

Meter (Pullman, WA), calibrated with a white standard tile (Y=98.8; x=0.3131, y=.3191). Color

was recorded using the CIE-L* a* b* uniform color space, where L* indicates lightness, a*

indicates hue on a green (−) to red (+) axis, and b* indicates hue on a blue (−) to yellow (+) axis.

The CIE L*, a* and b* values were subsequently used to determine the chroma (C*), hue angle

(Ho), and total color difference (ΔE*), as shown in Eq. 2.2 – 2.4.

∗ ∗ ∗ / (2.2)

°∗

∗ (2.3)

∆ ∗ ∆ ∗ ∆ ∗ ∆ ∗ / (2.4)

All analyses were performed in triplicate.

Estimation of concentration factor during FO

The concentration factor for the acid whey was determined using Eq. 2.5:

(2.5)

where CFt = concentration factor at time t (h);

Ct = total solids content of the feed solution at time t;

Ci = total solids content of the feed solution at the start of the experiment.

Since a linear relationship was obtained between ºBrix and total solids, the concentration

factor (CF) at time t was calculated using ºBrix instead of total solids content (Eq. 2.6):

38

(2.6)

where CFt = concentration factor at time t (h);

TSSt = total soluble solids content (ºBrix) at time t;

TSSi = total soluble solids content (ºBrix) of the feed at the start of the experiment.

2.3. Results and Discussion

Composition and color characteristics of acid whey before and after concentration

The composition of the acid whey feed is shown in Table 2.1, and a comparison between

the physicochemical analysis of acid whey and concentrates I and II, is shown in Table 2.2.

Table 2.1. Acid whey composition (Menchik, 2016)

Parameter Value Parameter Value

Titrable acidity 4.33 mg/g Total Protein 0.371%

Total solids 6.00% K 164 mg/100g

Total fat 0.00% Na 37.9 mg/100g

Ash 0.641% Ca 121 mg/100g

Moisture 94.56% Mg 10.6 mg/100g

Lactose 3.33% P 66.8 mg/100g

True protein 0.308%

39

Table 2.2. Composition of acid whey and concentrated acid whey.

Parameters Raw acid whey Concentrate I Concentrate II

ºBrix 6.5 31.4 40.4

Water activity 0.999 0.964 0.944

pH 4.41 (20ºC) 4.51 (20ºC) 4.46 (20ºC)

Conductivity (mS/cm) 6.62 12.05 10.22

Table 2.3 shows the color characteristics of raw acid whey, concentrate I, and concentrate

II. The total color difference (ΔE*) between raw acid whey and concentrate I, calculated using

Eq. 2.3, was determined to be 25.20, while ΔE* between concentrates I and II was 8.55.

Compared to raw acid whey, the concentrates had decreased values for lightness, and red hue

(a*), and an increased value for yellow hue (b*) and chroma (C*). The color changes were

slightly more pronounced in concentrate II than concentrate I compared to the initial whey, as the

former was had been concentrated to a greater extent.

Table 2.3. Color characteristics of acid whey and concentrated acid whey.

Raw acid whey Concentrate I Concentrate II

L* 91.46 79.55 72.04

a* -2.03 -7.14 -11.13

b* 7.50 29.11 29.96

C* 7.77 29.98 31.96

Ho -1.31 -1.33 -1.22

40

Variation in total soluble solids with total solids content

Figure 2.2 shows that, within the concentration range tested (7.1 and 44.6 ºBrix), the total

solids content of acid whey has a linear relationship with total soluble solids content (°Brix).

Figure 2.2. Relationship between total soluble solids (°Brix) and total solids content (%) for

skim Greek yogurt acid whey

Therefore, measuring the total soluble solids of concentrated acid whey presents a simple

and quick method for estimating its total solids content, by using Eq. 2.7:

TS = 1.027 TSS - 0.209 (2.7)

Concentration factor during FO of acid whey

Figures 2.3 and 2.4 illustrate the increase in TSS content with time during the FO

concentration of raw acid whey and pre-concentrated acid whey, respectively. During the

concentration of raw acid whey, the mean TSS content increased from 6.2 ± 0.1 ºBrix to 28.2 ±

1.7 ºBrix over 165 min. During the concentration of pre-concentrated acid whey, the mean TSS

content increased from 28.8 ± 0.7 ºBrix to 42.5 ± 3.3 ºBrix.

41

Figure 2.3. Variation in total soluble solids content during concentration of raw acid whey.

Values represent means ± standard error.

Figure 2.4. Variation in total soluble solids content during concentration of pre-concentrated

acid whey. Values represent means ± standard error.

0

5

10

15

20

25

30

0 30 60 90 120 150 180

°Brix

Time (min)

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25 30 35 40

°Brix

Time (min)

42

Flux

Figures 2.5 and 2.6 show the change in flux with time during the concentration of raw

and pre-concentrated acid whey, respectively. During the concentration of raw acid whey, the

water flux decreased from 8.67 ± 0.26 L/m2h to 5.52 ± 0.42 L/m2h. During the concentration of

pre-concentrated acid whey, the flux decreased from 5.04 ± 0.17 L/m2h to 2.93 ± 0.50 L/m2h.

The decline in flux with time was due to the increased feed concentration, which decreases the

driving force across the membrane. Figure 2.7 illustrates the relatively linear decline in flux with

increasing TSS content.

Figure 2.5. Flux during concentration of raw acid whey. Values represent means ± standard

error

0

1

2

3

4

5

6

7

8

9

10

0 30 60 90 120 150 180

Flux (L/m

2h)

Time (min)

43

Figure 2.6. Flux during concentration of pre-concentrated acid whey. Values represent means ±

standard error

Figure 2.7. A comparison of flux versus total soluble solids content during concentration of acid

whey.

0

1

2

3

4

5

6

7

8

9

10

0 5 10 15 20 25 30 35 40 45 50

Flux (L/m

2h)

TSS content (°Brix)

Raw acid whey

Pre‐concentrated acid whey

0

1

2

3

4

5

6

0 5 10 15 20 25 30 35 40

Flux (L/m

2h)

Time (min)

44

Concentration factor

Figures 2.8 and 2.9 illustrate the increase in concentration yield during the FO concentration

experiments. A final CF of 4.53 was obtained from the FO concentration of raw acid whey over

165 min (Figure 2.8). A final CF of 1.47 was obtained from the FO concentration of pre-

concentrated acid whey over 35 min (Figure 2.9).

Figure 2.8. Concentration yield during concentration of raw acid whey. Values represent means

± standard error.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0 30 60 90 120 150 180

Concentration factor 

Time (min)

45

Figure 2.9. Concentration yield during concentration of pre-concentrated acid whey. Values

represent means ± standard error.

Variation in feed temperature

Unlike evaporative concentration, membrane concentration processes allow better

retention of organoleptic and functional properties of feed components, as they involve low

temperatures. In addition, maintaining acid whey at cold temperatures helps prevent microbial

growth and spoilage.

Figures 2.10 and 2.11 show that the temperature of the feed solutions increased steadily

during the concentration of raw acid whey and pre-concentrated acid whey. This increase in

temperature, while not expected to cause any significant changes in product chemical and

microbiological quality, may have affected the water flux reported above. It was shown before,

that under similar conditions, that an increase in feed solution temperature causes an increase in

flux (Garcia-Castello, McCutcheon & Elimelech, 2009; Zhao & Zou, 2011).

To maintain the temperature of the feed solution during future experiments, a

recirculating cooler with a larger capacity or a jacketed feed solution vessel will be required.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0 5 10 15 20 25 30 35 40

Concentration factor

Time (min)

46

Figure 2.10. Temperature variation during concentration of raw acid whey. Values represent

means ± standard error.

Figure 2.11. Temperature variation during concentration of pre-concentrated acid whey. Values

represent means ± standard error.

0

2

4

6

8

10

12

14

16

18

20

0 30 60 90 120 150 180

Process tem

perature (°C)

Time (min)

0

2

4

6

8

10

12

14

16

18

0 5 10 15 20 25 30 35 40

Process tem

perature (°C)

Time (min)

47

2.4. Conclusions

In this study, FO was determined to be a feasible method for concentrating acid whey. Skim

Greek yogurt acid whey was concentrated to 42.5 ºBrix. Further work should be conducted to

determine the maximum degree of concentration that can be achieved. Additionally, the effect of

draw solution concentration and temperature on the flux should be studied, which will help to

determine efficient operating conditions for the process.

2.5. References

AOAC International. (2007). Official methods of analysis. Gaithersburg, MD: AOAC

International.

Arla. (2014). Greek yogurt process promises an end to 1 million tons of acid whey. Retrieved

from http://arlafoodsingredients.com/about1/talking-points/issue-43/greek-yogurt-process-

promises-an-end-to-1-million-tons-of-acid-whey/

Cath, T. Y., Childress, A. E., & Elimelech, M. (2006). Forward osmosis: principles, applications,

and recent developments. Journal of membrane science, 281(1), 70-87.

Ederna. (2016). Ederna lab unit user manual version 3.0. Toulouse, France.

Garcia-Castello, E. M., McCutcheon, J. R., & Elimelech, M. (2009). Performance evaluation of

sucrose concentration using forward osmosis. Journal of membrane science, 338(1), 61-66.

Ling, K. C. (2008). Whey to Ethanol: A Biofuel Role for Dairy Cooperatives? (Research Report

214). Retrieved from http://www.rd.usda.gov/files/RR214.pdf

Menchik, P. (2016). Acid whey composition. Unpublished raw data.

48

NYSDEC (New York State Department of Environmental Conservation). (n.d.) Whey

management for agriculture. Retrieved from http://www.dec.ny.gov/chemical/94164.html

Zhao, S., & Zou, L. (2011). Effects of working temperature on separation performance,

membrane scaling and cleaning in forward osmosis desalination. Desalination, 278(1), 157-164.

Tetra Pak. (2016). Dairy Processing Handbook. Available from

http://www.dairyprocessinghandbook.com/